linux-kernel - Re: [PATCH v2] MIPS: io: add a barrier after register read in readX()

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Message-ID: <58d1d3e5-bec8-1c34-e94a-c3c57321d6eb@codeaurora.org>
Date:   Tue, 3 Apr 2018 08:38:29 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "open list:RALINK MIPS ARCHITECTURE" <linux-mips@...ux-mips.org>,
        Timur Tabi <timur@...eaurora.org>, sulrich@...eaurora.org,
        linux-arm-msm@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Paul Burton <paul.burton@...s.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] MIPS: io: add a barrier after register read in readX()

On 4/3/2018 3:21 AM, Arnd Bergmann wrote:
> On Mon, Apr 2, 2018 at 8:13 PM, Sinan Kaya <okaya@...eaurora.org> wrote:
>> While a barrier is present in writeX() function before the register write,
>> a similar barrier is missing in the readX() function after the register
>> read. This could allow memory accesses following readX() to observe
>> stale data.
>>
>> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
>> Reported-by: Arnd Bergmann <arnd@...db.de>
>> ---
>>  arch/mips/include/asm/io.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/mips/include/asm/io.h b/arch/mips/include/asm/io.h
>> index 0cbf3af..7f9068d 100644
>> --- a/arch/mips/include/asm/io.h
>> +++ b/arch/mips/include/asm/io.h
>> @@ -377,6 +377,7 @@ static inline type pfx##read##bwlq(const volatile void __iomem *mem)        \
>>                 BUG();                                                  \
>>         }                                                               \
>>                                                                         \
>> +       war_io_reorder_wmb();                                           \
>>         return pfx##ioswab##bwlq(__mem, __val);                         \
>>  }
> 
> I'm not sure if this is the right barrier: what we want here is a read
> barrier to
> prevent any following memory access from being prefetched ahead of the readl(),
> so I would have expected a kind of rmb() rather than wmb().
> 

That's true. There was too much macro-ism in the code. I was thinking war_io_reorder_wmb()
to be a mb() under the hood. I'll fix and post an update.

> The barrier you used here is defined as
> 
> #if defined(CONFIG_CPU_CAVIUM_OCTEON) || defined(CONFIG_LOONGSON3_ENHANCEMENT)
> #define war_io_reorder_wmb()            wmb()
> #else
> #define war_io_reorder_wmb()            do { } while (0)
> #endif
> 
> which appears to list the particular CPUs that have a reordering
> write buffer. That may not be the same set of CPUs that have the
> capability to do out-of-order loads.
> 
>        Arnd
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.